Diffusion mechanisms of hydrocarbons within zeolites, especially MFI-type zeolite, were investigated by discriminating intracrystalline diffusivity from effective diffusivity. Intracrystalline diffusivity directly represents the mobility of molecules within pores. Effective diffusivity is obtained by multiplying the intracrystalline diffusivity by a partition factor given by the ration of the concentrations of molecules in zeolite crystals to that in gas phase. Intracrystalline diffusivity was the main subject of this study. Diffusion within MFI-type zeolite is dominated by the following mechanisms: (1) configurational diffusion, (2) resistance to mass transfer at pore mouths, (3) adsorption-controlled diffusion and (4) co-existing molecules with slow diffusing molecules. Intracrystalline diffusivity in the adsorption process is lower than in desorption process for (1) and (2) if the minimum molecular size is larger than the pore diameter, such as ortho- and meta-xylenes. The resistance to mass transfer at pore mouths becomes dominant in the adsorption process. This tendency is also observed for paraffins. Model equations were proposed for evaluating intracrystalline diffusivity based on the molecular size, molecular weight and pore diameter. Overall intracrystalline diffusivity for (3) is correlated with the configurational diffusion and the trapping effect on acid sites. This effect disturbs the mass transfer, especially at temperatures below 573 K, for aromatics. Similar effects are observed for lighter paraffins and olefins within MFI-type zeolites with metal cations. Intracrystalline diffusivities for (4) for silicalite-1 in a multicomponent system were measured using a new desorption under reduced pressure method. The diffusivity of the slow component in the multicomponent system agreed well for that of a single component. However, the diffusivity of the fast component was largely decreased by co-existing slow molecules. A random walk simulation and an empirical equation could explain this tendency. Effective diffusivity was calculated from the intracrystalline diffusivity and the partition factor, which was obtained from the adsorption isotherm. The partition factor suggested a marked condensation effect for MFI-type zeolites. Intracrystalline and effective diffusivities for beta- and Y-type zeolites, and mordenite were also investigated.
The present study was undertaken to improve the understanding of the movement behavior of water drops in the electro-desalter during crude petroleum refining. Fluid flows within and around a single spherical drop, which translates parallel to the applied electric field, were simulated using numerical analysis. The flows around the drop were governed significantly by the Reynolds number, viscosity ratio and a dimensionless parameter, which was defined by the effects of the electric field and gravitational field. The flows can be utilized for the removal of soluble salts from crude petroleum depending on the design of the electrical properties of the oil and water phases.
Ammonia adsorption isotherm on ion exchanged Y-zeolites (Na-Y, H-Y, Co-Y, Cu-Y, K-Y, Rb-Y, and Cs-Y) was investigated to asses the potential for use in ammonia separation and storage, by measuring the adsorption isotherm at 323 to 473 K and below 1 atm. Ammonia adsorption on Y-zeolite was increased by exchanging the cation with transition metal ions due to the increase in the number of ammonia adsorption sites with ammine complex formation, but was decreased by exchanging with alkali metal ions due to the decreased electrostatic attraction between ammonia and the zeolite surface. Irreversible ammonia adsorption sites on the ion exchanged Y-zeolite were classified into 3 types by IR (infrared) and TPD (temperature programmed desorption) techniques; M(OH)+ (M: divalent cation), H+, and M+ (M: alkali metal ion Na+, K+, Rb+, Cs+). The first type of site bonds by ammine complex formation, the second type of site bonds by ammonium ion formation, and the third type of sites bonds by ammonia adsorption with electrostatic attraction. Cu2+ exchanged Y-zeolite provided the best ammonia separation (4.92 mmol·g-1) with the temperature swing adsorption method (323-473 K, 40 kPa).
This study attempted to reproduce the particular morphological features of the carbonaceous material deposited on the shell wall of heat-exchangers in the commercial pyrolytic process converting ethylene dichloride (EDC) into vinyl chloride monomer to investigate the mechanism and factors affecting the deposition. Scanning electron microscope shows the carbonaceous material as an aggregate of nanospheres which coat the whole wall of the reactor. The effects of time and temperature in the heat-treatment were examined in an stainless steel reactor at 220-250°C using pure EDC and commercial feed EDC in the process. The yield of carbonaceous material from pure EDC increased almost proportionally with the period of heat-treatment from 0.12 wt% at 6 h to 0.73 wt% at 48 h. The aggregate of the spheres found on the wall also increased in diameter according to the period of the heat-treatment. The higher temperature increased both yield and size of the carbonaceous spheres. The feed EDC which contained chloro-olefins and dienes provided a much higher yield of carbonaceous deposition. Addition of 1% benzene to pure EDC at 250°C reduced the yield and particle size of deposited carbonaceous material. Addition of 1-methylnaphthalene to pure EDC produced a higher yield of carbonaceous particles than pure EDC at 250°C, maintaining the shape of the individual spheres. Biphenyl and tetralin slightly reduced the yield. Oxidized iron surface (Fe2O3) increased the yield, whereas glass surface reduced the yield of carbonaceous material. Dehydrochlorination, which occurs in this temperature range, appears to increase the yield of carbonaceous material. Based on the morphology and yield of deposited carbonaceous material, the mechanism and reduction of carbonaceous deposition are discussed.
Naphthalene hydrogenation was carried out over TiO2- and Al2O3-supported bimetallic Pt-Pd catalysts at 0.95-2.45 MPa and 473 K with and without addition of dimethyldisulfide. The bimetallic catalysts were characterized by infrared spectroscopy of the adsorbed CO. Compared with the Pt-supported catalysts, the Pd-supported catalysts had higher catalytic activity in the presence of sulfur. Pd catalysts had a higher sulfur-tolerance, because the catalytic activity of the Pt catalysts was higher than that of the Pd catalysts in the absence of sulfur. Coexistence of Pd with Pt induced significant synergy in the catalytic properties in the presence of sulfur. However, such synergy was not observed in the absence of sulfur. The optimum Pd/(Pt + Pd) molar ratio for the Al2O3- and TiO2-supported catalysts was 0.8 and 0.5, respectively. The difference was an effect of metal particle size. Infrared spectroscopy of the adsorbed CO on the bimetallic catalysts showed the formation of Pt-Pd bimetallic particles. However, no electronic interaction between Pt and Pd was observed. Therefore, the synergy was due to the geometric effect.
Conventional methods for biomass gasification to hydrogen and synthesis gas are carried out at high temperature (1073-1673 K). Recently Rh/CeO2/SiO2 catalyst has been shown to be very effective for the catalytic gasification of biomass at low reaction temperatures. The catalytic performance of the Rh/CeO2/SiO2 catalyst for the pyrogasification of cedar wood biomass was compared with that of a commercial steam reforming Ni catalyst. The Ni catalyst allowed the gasification of tar but not coke during the pyrogasification at 873-1023 K. In contrast, Rh/CeO2/SiO2 gasified part of coke as well as tar. Testing of these catalysts for steam and CO2 reforming of biomass found that steam reforming proceeded more easily than CO2 reforming of methane over both catalysts.
The combinatorial approach is widely used for homogeneous and heterogeneous catalyst development. The main key technologies are “combinatorial chemistry (CC)” for material preparation and “high-throughput screening (HTS)” for rapid assay using automated and/or robotic equipment. A HTS reactor with 96 parallel lines was designed and manufactured to optimize the Cu-Zn catalyst for methanol synthesis. A neural network (NN) was constructed from the “catalyst composition-activity” dataset obtained by the HTS reactor. The catalyst composition was optimized by a genetic algorithm combined with the trained NN. Active Cu-Zn catalysts for methanol synthesis under CO2 rich syngas were discovered by these combinatorial tools.
A silica with bimodal pore structures was prepared by a simple method of introducing silica sol into the controlled large pores of original silica gel. The pores of the bimodal pore silica were distributed into two distinct pore sizes that could be accurately controlled. The increased BET surface area and the decreased pore volume of the obtained bimodal pore supports, as compared to those of the original silica gel, indicated that the particles of silica sols formed small pores inside the original large pores of silica gel, resulting in formation of the bimodal pore structure silica support. Both pore sizes could be controlled, as the large pores originated from tail-made pellet while mesopore size was equal to the diameter of sol particles regardless of sol concentration during catalyst support preparation.
A series of metal-promoted 20 wt%Co-0.5 wt%Ir-SiO2 catalysts was prepared by the alkoxide method, and the effects of additives (MOx; M = K, Cr, Al, Ce, La, and Mn) on the catalysis of Fischer-Tropsch reaction were investigated. K-Promoter reduced CO conversion. Al- or Cr-promoter resulted in almost the same CO conversion as over the catalyst without additives, but CH4 selectivity increased. La- or Ce-promoter decreased C5+ selectivity. In contrast, CO conversion and the α-value (the chain growth probability of CHx intermediates) increased up to 57% and 0.85, respectively, over catalysts with 10 wt% Mn promoter. However, further addition of Mn caused deactivation of the catalyst. Selectivity for CH4 showed a linear relationship with the standard enthalpy of formation (-ΔHf0) of the promoter oxides (MOx).